Experimental Analysis of Thermal Runaway in 18650 Cylindrical Li-Ion Cells Using an Accelerating Rate Calorimeter

In this work commercial 18650 lithium ion cells with LiCoO2 and LiMn2O4 cathodes were exposed to external heating in an Accelerating Rate Calorimeter (es-ARC, THT Company) in order to investigate the thermal behavior under abuse conditions. During the experiments the temperature and the internal pressure changes of the cells were measured in the ARC. Furthermore, external pressure changes were measured in order to detect venting of the cells. Before the measurement the cells were fully charged and the so-called Heat-Wait-Seek method was applied. This procedure starts by heating the sample in small incremental steps. At the end of each step the calorimeter “waits” if the battery is generating heat that can be measured by a temperature rise (so called "seek" step). When self-heating of the cell is detected, the ARC switches to the adiabatic mode in which the temperature of the calorimeter follows instantaneously the surface temperature of the cell, i.e. the cell cannot exchange heat with the surroundings anymore. This procedure is repeated until a thermal runaway occurs. For the external pressure measurements, the cell was placed in a gas-tight cylindrical compartment inside the calorimeter chamber. This compartment was equipped with an outlet where a pressure line is connected with a pressure sensor. In contrast to the external pressure measurement for internal pressure measurements this pressure line was directly inserted into the cell. Therefore, a suitable place had to be found by using images from X-Ray Tomography. In a second experiment a current profile was applied to the cell with maximal C-rate and cycled until a thermal runaway occurs. During these experiments again the internal and external pressure was measured as well as the cell temperature. In addition the heat capacity was measured to calculate the heat of reaction. From the results of the thermal runaway experiments, three stages (low rate reactions stage, medium rate reactions stage and high rate reactions stage) have been observed. Both pressure and temperature change indicated different stages of exothermic reactions, which would produce gases or/and heat. The onset temperature of thermal runaway was estimated according to temperature and pressure changes in the measurements. Moreover, the different activation energies for the exothermic reactions could be derived from Arrhenius plots. Finally, the experimental data were compared with the results of numerical simulations based on an electrochemical-thermal model which was extended with additional source terms in order to describe the exothermic reactions inside the cell during thermal runaway. The experimentally determined activation energies have been used as input parameters for these simulations. It could be shown that the above mentioned reactions stages can also be found in the simulations.